Production of chlorine dioxide

ABSTRACT

Chlorine dioxide is produced from three reactors, the first reacting sodium chlorate, sodium chloride and/or hydrogen chloride, and sulphuric acid at an acidity in excess of 4.8N to deposit sodium acid sulphate, the second reacting sodium chlorate, sodium chloride and/or hydrogen chloride, and a sulphate-ion material which is constituted at least in part by the sodium acid sulphate deposited in the first reaction, at an acidity greater than 4.8N to deposit sodium sesquisulphate. In the third vessel sodium chlorate, sodium chloride and/or hydrogen chloride and a sulphate-ion material which is constituted at least in part by the sodium sesquisulphate at an acidity less than 4.8N to deposit sodium sulphate.

United States Patent [191 Rapson PRODUCTION OF CHLORINE DIOXIDEInventor: William Howard Rapson,

Scarborough, Ontario. Canada Assignee: ERCO Industries Limited,lslington,

Ontario, Canada Filed: Feb. 8, 1973 Appl. No.: 330,602

Related U.S. Application Data Division of Ser. No. 147,821, May 28,1971.

[52] U.S. Cl 423/478, 423/477, 423/520,

423/351, 423/552, 252/187 R Int. Cl...... C0lb 11/02, COld 5/02, COlh7/02 Field of Search... 423/478, 511, 520, 551, 552;

Primary Examiner-Edward Stern [5 7] ABSTRACT Chlorine dioxide isproduced from three reactors, the first reacting sodium chlorate, sodiumchloride and/or hydrogen chloride, and sulphuric acid at an acidity inexcess of 4.8N to deposit sodium acid sulphate, the second reactingsodium chlorate, sodium chloride and/or hydrogen chloride, and asulphate-ion material which is constituted at least in part by thesodium acid 252/187 R sulphate de sited in the first reaction, at anacidity th 4 8N d d' l h l greater an to eposltso lum sesqulsup ate. n{56] References cued the third vessel sodium chlorate, sodium chlorideand- UNITED STATES PATENTS [or hydrogen chloride and a sulphate-ionmaterial 3,341,288 9/1967 Partridge et all. 423/478 which is constitutedat least in part by the sodium ses- 3.3 7,628 0/1967 Sepall et a 252/18R X quisulphate at an acidity less than 4.8N to deposit so- 3,563,702 21971 Partridge et all 423/478 dium su1phate FOREIGN PATENTS ORAPPLICATIONS 543,589 7/1957 Canada 423/478 4 Claims, 1 Drawing Figure118 ClO /Cl P 13 152 .1146 Nacl O 44 NaCL 142 -15C NaClO Twacto 11 12s128 1138 140 l 120 GENERATOR 132 LGENERATDR GENERATOR 114 L124 1 l36 1 i22 [13 1 144 4 hi ooa Na SO PRODUCTION OF CHLORINE DIOXIDE Thisapplication is a division of U.S. Application Ser. No. l47,821 filed May28, 1971.

This invention relates to the production of chlorine dioxide.

Chlorine dioxide is used in the bleaching or cellulosic materials, suchas wood pulp produced by the Kraft or sulfite processes.

One of the known methods of production of chlorine dioxide involvesreaction of an alkali metal chlorate, an alkali metal chloride andsulphuric acid. The reactions involved are as follows:

1. MClO MCl H 80 ClO, %Cl M SO H 2. MClO SMCl BH SO 3Cl 3M,S0 3H,0 whereM is an alkali metal. Reaction 2 becomes significant when the mole ratioof chloride to chlorate substantially exceeds lzl. Since no chlorinedioxide is produced by the process of equation 2, to maximize productionof chlorine dioxide from chlorate, it is preferred to operate at anapproximately equimolar or only slightly higher ratio of chloride tochlorate in the feed.

A method of operating the above system to produce chlorine dioxide andchlorine for use in bleaching, for example, wood pulp, and usedcommercially is known as the Rapson R2 process, also known as the HookerR2" and ER2" processes, as disclosed in Canadian Pat. No. 543,589,issued Jul. 16, 1957 to Electric Reduction Co. of Canada, Limited. TheRapson R2 process involves introduction into a generator of a stream ofan aqueous solution of an approximately equimolar or slightly higherproportion of sodium chloride and sodium chlorate and a stream ofconcentrate sulphuric acid. Air is introduced at the bottom of thegenerator to carry chlorine dioxide and chlorine produced in thegenerator out of the generator and to further processing to separate thechlorine dioxide and chlorine. The aqueous effluent from the generatorcontains sodium bisulphate (NaHSO and unreacted sodium chlorate, sodiumchloride and sulphuric acid. The sodium bisulphate may be crystallizedout and the mother liquor returned to the generator. The sodiumbisulphate may be mixed with sodium sesquisulphate (Nag-[(500 dependingon the conditions of operation of the generator.

The R2 process generally is carried out under conditions of highacidity, such as about ION. In accordance with an invention described inCanadian Pat. No. 826,577 issued Nov. 4, 1969, to Electric ReductionCompany of Canada, Limited, chlorine dioxide and chlorine can beobtained by reacting sodium chlorate, sodium chloride and sulphuric acidunder conditions of low acidity, generally of the order of 2 to 4.8 N.

The effluent from the generator used in this low acidity operationcontains sodium sulphate (Na SO in addition to unreacted sodiumchlorate, sodium chloride and sulphuric acid. The sodium sulphate may berecovered by either evaporating the solution, in which case the sodiumsulphate is recovered as anhydrous sodium sulphate, or by cooling thesolution, in which case the sodium sulphate is recovered as sodiumsulphate decahydrate (Na,SO.-1OH,O). The mother liquor then may berecycled to the generator.

In Canadian Pat. No. 825,084 issued Oct. 14, 1969 to Electric ReductionCo. of Canada, Limited, there is described a process of forming chlorinedioxide and chlorine from sodium chlorate, sodium chloride and sulphuricacid in which the chlorine dioxide and chlorine are generated and asodium acid sulphate crystallized in the same vessel. This may beachieved by operating the generator at the boiling point of thereactants under reduced pressure. The water evaporated thereby serves toremove the chlorine dioxide and chlorine generated from the vessel,thereby eliminating the necessity of a separate air stream to remove thegaseous products from the vessel. In addition, the quantity of water isreduced in the vessel as the evaporation takes place, thereby inducingprecipitation of the sodium acid sulphate within the vessel.

The sodium acid sulphate is recovered from the vessel and the spentreaction liquor containing unreacted sodium chlorate, sodium chlorideand sulphuric acid may be returned to the generator.

The form of the sodium acid sulphate produced is dependent on theacidity and the temperature of the reacting liquor. The acid sulphatemay be in the form of sodium bisulphate, i.e., Nal-lSO or sodiumsesquisulphate Nag-RS03 Sodium sesquisulphate may be produced over anacidity range of 4.8 to 9N. For example, while sodium bisulphate may beproduced at an acidity of ION at about C in boiling solution, sodiumsesquisulphate may be produced at 8N acidity at about 30C in boilingsolution.

The process disclosed in the aforementioned Canadian Pat. No. 826,577and described above preferably is performed in a single vessel, whereinchlorine diox ide and chlorine are generated, water is evaporated andsodium sulphate is precipitated. Under the conditions of low acidity,i.e., about 2 to about 4.8N, the sodium sulphate deposited is generallyanhydrous sodium sulphate (Na SO possibly mixed with some sodiumsesquisulphate at the high end of the acidity range.

In accordance with the present invention, three sin gle vesselgenerator-evaporator-crystallizers are operated in sequence, twooperating at decreasing high acidity and one at low acidity.

Single vessel generator-evaporator-crystallizers suitable for use in theprocess of the present invention are described in the abovementionedCanadian Pat. No. 825,084.

A three-stage chlorine dioxide production operation in accordance withthis invention comprises a first stage wherein sodium chlorate, achloride which is sodium chloride, hydrochloric acid or mixtures thereofand sulphuric acid is reacted in aqueous medium in a single vesselgenerator-evaporator-crystallizer at an acidity in the region of 9 toI2N. Sufficient water is evaporated from the aqueous medium to inducecrystallization of sodium bisulphate. l

The evaporated water acts as a diluent for the chlorine dioxide andchlorine generated in the reaction, and the product gases are removedfrom the vessel as a gaseous mixture including the steam.

The sodium chlorate and sodium chloride may be in troduced into thevessel as aqueous solutions or if desired, in dry form. Any desiredmolar ratio of sodium chloride: sodium chlorate can be employed, but itis preferred to utilize an approximately equimolar ratio in order tomaximize the production of chlorine dioxide in accordance with equation1 above. Ratios of chloride: chlorate generally vary between 1:1 and3:1, preferably about 1.1:] to 1.3:1.

An aqueous solution containing both sodium chlorate and sodium chloridemay be fed as one stream to the vessel and the sulphuric acid as aseparate stream. Alternatively, aqueous solutions of sodium chlorate andsodium chloride may be fed as separate streams to the vessel.

The sulphuric acid generally is added to the vessel in concentrated formin order to produce the required acidity in the reaction medium.

Concentrations of chlorate and chloride in the mac tion medium may varyover a wide range. For example, the concentration of the chlorate in thereacting solution may be in the range of about 0.005 to about 3 molarand the concentration of the chloride may be in the range of about 0.001to about 2 molar.

The temperature of the reaction medium may vary over a wide range, butis generally between about 30 and 80C. It is preferred to operate thevessel at substantially the boiling point of the reaction medium,whereby maximum rate of evaporation of water vapour is achieved. Tomaintain the reaction medium at its boiling point, the vessel preferablyis subjected to an at least partial vacuum.

Chlorine dioxide gas at normal atmospheric pressure spontaneouslydecomposes with detonation. The water vapour dilutes the chlorinedioxide rendering it less susceptible to spontaneous decomposition. Thechlorine dioxide and chlorine produced are removed from the generator asa mixture with the evaporated water.

The sodium bisulphate crystallized in the vessel is separated from theliquor and transferred in accordance with this invention to a secondsingle vessel generator-evaporator-crystallizer containing an aqueousreaction medium comprising sodium chlorate and a chloride which issodium chloride, hydrochloric acid or a mixture thereof.

The sodium bisulphate supplies at least a substantial proportion,preferably all, of the acid requirement of the second vessel due to itsacidic nature. The second vessel operates at a lower acidity, generallyfrom about 5 to 9N. Any additional acid requirement may be provided bysulphuric acid. Sufficient water vapour is evaporated from the reactionmedium to crystallize sodium sesquisulphate (Nag- (80 in the vessel.

The separated sodium sesquisulphate crystallized in the vessel isseparated from the liquor and is transferred to a third single vesselgenerator-evaporatorcrystallizer containing an aqueous reaction mediumcomprising sodium chlorate and a chloride which is sodium chloride,hydrochloric acid or a mixture thereof. Te sodium sesquisulphatesupplies at least a substantial proportion, preferrably all of the acidrequirement of the third vessel due to its acidic nature. The thirdvessel, therefore, operates at a low acidity, generally from about 2 to4.8N. Any additional acid requirement may be provided by sulphuric acid.Sufficient water vapour is evaporated from the reaction medium tocrystallize anhydrous sodium sulphate (Na- S0.) in the vessel.

Where all of the sodium bisulphate formed in the first stage is used asthe sole acid source of the second stage and all of the sodiumsesquisulphate formed in the second stage is used as the sole acidsource of the third stage, the reactions involved are represented by thefollowing equations:

2NaClO3 ZNaCl 4H2SO4 2ClO-2 Cl H2O l' Naclog l i a 2NZ13H(SO4)2 'i' H20NaClO i a "l- 4Na SO lherefore, half of the total chlorine dioxideproduced by the overall process is formed at high acidity utilizingsulphuric acid, one-quarter of the total chlorine dioxide is formed athigh acidity using sodium bisulphate as the acid and one-quarter of thetotal chlorine dioxide is formed at low acidity using sodiumsesquisulphate as the acid.

The high acidity reactions are about 99 percent efficient but involveacid losses whereas the low acidity reaction is about 94 percentefiicient and involves no acid losses. The overall process, therefore,is about 97.8 percent efficient for conversion of sodium chlorate tochlorine dioxide and involves no acid losses.

The evaporated water acts as a diluent for the chlorine dioxide andchlorine generated and the product gases are removed from the second andthird vessels by the steam.

As in the case of the first vessel, the sodium chlorate and sodiumchloride may be fed to the second and third vessels as a single streamof aqueous solution, or as separate streams of aqueous solutions of thesodium chlorate and sodium chloride. Concentrations of the sodiumchlorate and sodium chloride, temperature of reaction and otherparameters are similar to those de scribed above for the first vessel.

In particular, it is preferred that the mole ratio of chloridezchloratebe approximately I :l, the reaction be carried out substantially at theboiling point of the reaction medium and that the second and thirdvessels be maintained under a reduced pressure.

It is preferred that the process of the present invention be carried outcontinuously, so that sodium bisul phate recovered from the first vesselis transferred continuously to the second vessel, the sodiumsesquisulphate recovered from the second vessel is transferredcontinuously to the third vessel, and anhydrous sodium sulphate isrecovered continuously from the third vessel.

In the preferred embodiment wherein the process is operatedcontinuously, the level of the reaction medium in each vessel ismaintained substantially constant. Any overflow liquid from the firstvessel is recycled to the reactant feed input of the first vessel, anyoverflow liquid from the second vessel is recycled to the reactant feedinput of the second vessel, and any overflow liquid from the thirdvessel is recycled to the reactant feed input of the third vessel.

It is possible to combine this system with other chlorine dioxidegenerators. For example, a further generator in which chlorate, chlorideand sulphuric acid are reacted at high acidity, for example, lON may beprovided.

This latter generator may be operated so that no acid sulphate isdeposited in the vessel. The whole of liquid effluent from the generatormay be passed to the high acidity generator precipitating sodiumbisulphate. The effluent from one high acidity generator thereby formspart of the chlorate, chloride and acid requirement of another highacidity generator.

A number of chlorine dioxide generators may be operated in a seriesoperation in this way with passage of liquid effluent from one generatorto the next. It is essential, of course, that the last two high aciditygenerators in the series, prior to the low acidity generator, beoperated to precipitate acid sulphate for use as at least part of theacid requirement in successive generators.

These additional generators may be of the type described above whereinsteam is used to remove chlorine dioxide and chlorine from thegenerators, or of the R2-type mentioned above, using air as the dilutinggas.

Further, an additional single vessel-type high acidity chlorine dioxidegenerator may be provided in parallel operation with the existing highacidity generators. Such additional generator may be operated toprecipitate acid sulphate. The acid sulphate from this additiional highacidity generator may be forwarded to the low acidity chlorine dioxidegenerator as part of the acid requirement thereof.

In addition, where such generators are provided in parallel, one or morefurther high acidity generators of the steam dilution-type or RZ-typemay be provided. These latter generators may be operated so that acidsulphate is not precipitated therein, and the liquid effluent therefrommay be passed, partly to one and partly to the other of the parallel,acid sulphateprecipitating generators. Other combinations are possibleas will be evident to the skilled practitioner.

Such combinations may be desirable where large quantities of chlorinedioxide is required and only limited capacity generators are available.

The anhydrous sodium sulphate recovered from the low acidity reactionmay be used as a source of sodium and sulphur make-up in a Kraftrecovery system. In the Kraft process for the production of cellulosicfibrous pulp, the fibrous cellulosic material, generally wood chips, isdigested by heating with a white liquor containing sodium sulphide andsodium hydroxide to dissolve from the wood chips a substantial part ofthe hemicelluloses and the liquor and other extractable organicmaterials contained therein. The fibrous pulp so produced is separatedfrom the resulting black liquor and washed and passed to bleachingoperations.

The black liquor is subjected to a series of operations in a recoverysystem. The black liquor first is concentrated by evaporation of waterand the concentrated black liquor is burnt in a furnace to yield a smeltcontaining sodium carbonate and sodium sulphide. The smelt is dissolvedin water to yield a raw green liquor" which then is clarified. Theclarified green liquor is causticized with lime, whereby the sodiumcarbonate is converted into sodium hydroxide and calcium carbonate isprecipitated as a mud. The mud is calcined after washing to regeneratelime for further causticization. The causticized green liquor then isrecycled as white liquor.

To make up sodium and sulphur values lost from the system, sodiumsulphate is added, generally to the black liquor before it is fed to thefurnace. The sodium sulphate is converted in the furnace to form sodiumsulphide and sodium carbonate, the sodium carbonate being converted tosodium hydroxide on causticization. Thus, the sodium sulphide and sodiumhydroxide content of the white liquor is maintained at the desiredlevel.

Alternatively, part or all of the anhydrous sodium sulphate recoveredcould be converted into sulphuric acid by dissolving the sodium sulphatein a small amount of water and treating the solution with dry hydrogenchloride, in accordance with the invention described and claimed incopending Canadian Application Ser. No. 072,527 filed Jan. 20, 1970.

The invention is further described with reference to the accompanyingdrawing which is a flow sheet of one embodiment of the invention.

Referring to the drawing, sodium chlorate solution is fed through linesand 112 to a first generator 114, containing a boiling aqueous reactionmedium of sodium chlorate, sodium choride and sulphuric acid having ahigh acidity, typically around 10 N. Aqueous sodium chloride solution isfed to the first generator 114 through line 116 and concentratedsulphuric acid is fed through line 118. The generator 114 is maintainedunder a reduced pressure in any convenient manner. Any overflow liquorfrom the reaction medium is recycled by line 120 to line 112.

While the sodium chlorate and sodium chloride are indicated to be fed inseparate streams to the generator 114, in practice, there may be only asingle stream containing these two reactants.

The rate of feed of the feed streams, the rate of removal of liquor asoverflow and the rate of removal of water by evaporation are adjusted sothat the level of the reaction liquor in the generator 114 is maintainedsubstantially constant.

Water is evaporated from the reaction medium and sodium bisulphatecrystallizes out of the reaction medium. The crystallized sodiumbisulphate is fed by line 122 to a second chlorine dioxide generator124.

The generator 124 contains a boiling aqueous reaction medium of sodiumchlorate, sodium chloride and acid values. Feeds of aqueous sodiumchlorate and so dium chloride to the generator 124 may be made by lines126, 128 and 130. These feed streams may be combined into a singlestream if desired.

The generator 124 is maintained under a reduced pressure. Any overflowliquor from the reaction medium in the generator 124 is recycled by line132 to the line 128.

The sodium bisulphate in line 122 constitutes the sole source of acid inthe generator 124. By appropriate adjustment of concentration, thereaction medium in the generator 124 has an acidity of about 7 to 8N. Itmay be desired to supplement the acid feed from the bisulphate withsulphuric acid.

The rate of feed of the streams, the rate of removal of liquor asoverflow and the rate of removal of water by evaporation are adjusted sothat the level of the reaction liquor in the generator 124 is maintainedsubstantially constant.

Water is evaporated from the reaction medium and sodium sesquisulphatecrystallizes out of the reaction medium in generator 124 and is fed byline 134 to a third chlorine dioxide generator 136.

The generator 136 contains a boiling aqueous reaction medium of sodiumchlorate, sodium chloride and acid values. Feeds of aqueous sodiumchlorate and sodium chloride to the generator 136 may be made by lines138, 140 and 142. A single feed stream containing sodium chloride andsodium chlorate may be used, if desired.

The generator 136 is maintained under a reduced pressure. Any overflowliquor from the reaction me- 7 dium in the generator 136 is recycled byline 142 to the line 140.

The sodium sesquisulphate in line 134 constitutes the sole source ofacid in the generator 136 and imparts to the reaction medium an acidityof between 2 to 4.8 N. It may be desired to supplement the acid feedfrom the sesqusulphate with sulphuric acid.

The rate of feed of the streams, the rate of removal of liquor asoverflow and the rate of removal of water by evaporation are adjusted sothat the level of the reaction liquor in the generator 136 is maintainedsubstantially constant.

Water is evaporated from the reaction medium in the generator 136 andanhydrous sodium sulphate crystallizes out of the reaction medium. Theanhydrous sodium sulphate is removed from the generator 136 by line 144,for use in a Kraft mill or for further processing.

The gaseous products of the generators 114, 124 and 136 are removedrespectively by lines 146, 148 and 150 and form a common stream 152which may be passed to a chlorine dioxide absorber for separation of thechlorine dioxide and chlorine, as described above with reference to theembodiment of FIG. 1.

The generators 114, 124 and 136 may be of any convenient type, typicallyone described in the abovementioned Canadian Pat. 825,084.

The sodium chlorate fed to the generators 114, 124 and 136 may beprovided in any convenient manner, for example, from a chlorate cellwherein an acid solution of sodium chloride is electrolyzed. The sodiumchlorate may be dry fed, if desired.

The sodium chloride fed to the generators 114, 124 and 136 may bereplaced at least in part by hydrochloric acid.

The generators 114, 124 and 136 as mentioned are maintained at theboiling point of the reaction medium at a reduced pressure, typically ata temperature of about 75C.

The present invention provides a considerable advantage over theproduction of chlorine dioxide separately at high and low acidities.

Thus, the present invention has the advantage over the process carriedout at high acidity in a single vesselgenerator-evaporator-crystallizer, in that the acid values present inthe sodium acid sulphate crystallized from the generator are utilizedand not lost from the system. The present invention has advantage overthe process carried out at low acidity in a single vesselgenerator-evaporator-crystallizer, in that at least part of thesulphuric acid requirement can be supplied by a byproduct acid material,which would otherwise not have its acid values recovered.

The only solid product of the process of the present invention isanhydrous sodium sulphate which is readily utilizable in the Kraft millrecovery process described above, or in the production of sulphuricacid, in accordance with the process of copending Canadian Appli cationSer. No. 072,527.

Further, a large quantity of water is required to be evaporated in thelow acidity generator. By operating in accordance with the presentinvention, this quantity is reduced for the same overall quantity ofchlorine dioxide and anhydrous sodium sulphate produced, so that theheat requirement of the system is reduced. In addition, the efficiencyof conversion of sodium chlo rate to chlorine dioxide by the low acidityprocess is increased by use of the process of the present invention, bycontrast with operation of a single vesselgeneratorevaporator-crystallizer at low or high acidity.

Modifications are possible within the scope of this invention.

What I claim is:

l. A process for the production of chlorine dioxide which comprisesgenerating chlorine dioxide and chlorine from a first aqueous reactionmedium having an acidity from 9N to 12N in a first reaction zone, saidfirst aqueous reaction medium comprising sodium chlorate, a chlorideselected from sodium chloride, hy drogen chloride and mixtures thereof,and sulphuric acid, evaporating sufficient water from said firstreaction medium and subjecting said reaction medium to an elevatedtemperature to precipitate sodium bisulphate in said first zone,recovering said sodium bisulphate from said first zone, forming a secondaqueous reaction medium having an acidity from 5 to 9N in a second re'action zone by feeding to an aqueous solution contain ing sodiumchlorate and a chloride selected from sodium chloride, hydrogen chlorideand mixtures thereof, a sulphate ion-containing acidic materialcomprising at least part of said sodium bisulphate recovered from saidfirst zone, said at least part of said sodium bisulphate constituting atleast a major proportion of said sulphate ion-containing acidicmaterial, generating chlorine dioxide and chlorine from said secondreaction medium in said second reaction zone, evaporating sufficientwater from said second reaction medium and subjecting said reactionmedium to an elevated temperature to precipitate sodium sesquisulphatein said second zone, recovering said sodium sesquisulphate from saidsecond zone, forming a third aqueous reaction medium having an acidityfrom 2 to 4.8N in a third reaction zone by feeding to an aqueoussolution containing sodium chlorate and a chloride selected from sodiumchloride, hydrogen chloride and mixtures thereof, a sulphateion-containing acidic material comprising at least part of said sodiumsesquisulphate recovered from said second reaction zone, said at leastpart of said sodium sesquisulphate constituing at least a majorproportion of said latter sulphate ion containing acidic material,generating chlorine dioxide and chlorine from said third reaction mediumin said third reaction zone, evaporating sufficient water from saidthird reaction medium to precipitate anhydrous sodium sulphate in saidthird reaction zone, and recovering chlorine dioxide and chlorinegenerated in said first, second and third reaction zones.

2. The process of claim 1 wherein said first, second and third reactionmedia are maintained at a boiling temperature and said first, second andthird reaction zones are maintained under reduced pressure.

3. The process of claim 2 wherein the sodium bisulphate precipitated insaid first reaction zone constitutes the sole source of acidity of saidsecond reaction medium and the sodium sesquisulphate precipitated insaid second reaction zone constitutes the sole source of acidity of saidthird reaction medium.

4. The process of claim 1 wherein the acidity of said first reactionmedium is appoximately ION and the acidity of said second reactionmedium is approximately 7 to 8N.

i k =0 t

2. The process of claim 1 wherein said first, second and third reactionmedia are maintained at a boiling temperature and said first, second andthird reaction zones are maintained under reduced pressure.
 3. Theprocess of claim 2 wherein the sodium bisulphate precipitated in saidfirst reaction zone constitutes the sole source of acidity of saidsecond reaction medium and the sodium sesquisulphate precipitated insaid second reaction zone constitutes the sole source of acidity of saidthird reaction medium.
 4. The process of claim 1 wherein the acidity ofsaid first reaction medium is appoximately 10N and the acidity of saidsecond reaction medium is approximately 7 to 8N.